In a first conventional method for storing data which should be re-written a considerable number of times and should be maintained even in a state where the power supply is cut off, the data are stored in a volatile type memory such as a random access memory (RAM) which is backed up by a battery to maintain the data. The data can be re-written an unlimited number of times.
In a second conventional method for storing data which should be re-written a considerable number of times and should be maintained even in a state where the power supply is cut off, the data are stored in a non-volatile type memory such as an erasable programmable read only memory (EPROM) which maintains the data without backing up even in a state where the power supply is cut off.
According to the first and second conventional methods for storing data, however, there are disadvantages as described below.
In the first conventional method for storing data, the data can not be maintained for a long time in the state where the power supply is cut off, because the data may be volatilized if the battery is exhausted.
In the second conventional method for storing data, the number of times of re-writing is limited because of the property of the non-volatile type memory, so that there occurs a practical problem if the number of necessary times of re-writing is larger than that of possible times of re-writing of the non-volatile type memory.